Acquisition and Development Agreement (Details) (USD $)	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Mitsui E&P Texas LP	Sep. 30, 2011 Mitsui E&P Texas LP
Acquisition and development agreement
Percentage of working interests transferred to acquiree entity (as a percent)				12.50%
Acreage of working interests transferred to acquiree entity (in acres)				39,000
Percentage of the entity's costs and expenses during the first three years following the closing of the transaction, borne by the acquiree entity (as a percent)				90.00%
Costs and expenses incurred by acquiree entity on behalf of entity that the acquiree entity has agreed to pay				$ 680,000,000
Acquisition and Development Agreement, Estimated Time to Complete, Low End of Range	2
Acquisition and Development Agreement, Estimated Time to Complete, High End of Range	3
RetainedShareOfDrillingAndCompletionCosts	10.00%
Restricted Cash and Cash Equivalents, Noncurrent	93,771,000	124,703,000
Utilization Amount of Carry and Earning Agreement			$ 201,300,000